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Sunday, 26 June 2016

安装meek-server，用以翻墙

How to run a meek-server (meek bridge):
- Compile the program using 'go build'.
- Update your torrc file. There's a sample on /meek-server/torrc.
NOTE: if you want to run your bridge on two different ports (HTTP and HTTPS), use something like this:
ServerTransportPlugin meek exec /usr/local/bin/meek-server --port 7002 --disable-tls --log /var/log/tor/meek-server.log
ServerTransportPlugin meek exec /usr/local/bin/meek-server --port 7443 --cert /etc/meek/cert.pem --key /etc/meek/key.pem --log /var/log/tor/meek-server-https.log
- To test your bridge on the clinet side, you can add a line like this to your torrc:
Bridge meek 0.0.2.0:3 url=http://my-bridge.example.com:7002/
# Important Note:
If you're running more than one transport, you need a separate tor process for each, to avoid user counting confusion.
For more information, see https://lists.torproject.org/pipermail/tor-dev/2014-September/007480.html and https://trac.torproject.org/projects/tor/wiki/doc/meek#Users

from https://github.com/arlolra/meek/tree/master/meek-server

(meek-server is the server transport plugin for the meek pluggable transport. It acts as an HTTP server, keeps track of session ids, and forwards received data to a local OR port.

meek is a blocking-resistant pluggable transport for Tor. It encodes a
data stream as a sequence of HTTPS requests and responses. Requests are
reflected through a hard-to-block third-party web server in order to
avoid talking directly to a Tor bridge. HTTPS encryption hides
fingerprintable byte patterns in Tor traffic.
https://trac.torproject.org/projects/tor/wiki/doc/meek
The key trick that makes the system work is "domain fronting":
communicating with a forbidden domain in a way that makes it look like
you are communicating with an allowed domain. It works by putting the
allowed domain on the "outside" of a request: in the DNS query and the
SNI TLS extension; and the forbidden domain on the "inside": in the Host
header of the HTTP request. The trick works with web services that
ignore the SNI and handle requests based on the Host header. Google,
with its App Engine infrastructure at appspot.com, is one of these
services. A client wanting to communicate with a forbidden subdomain of
appspot.com while appearing to communicate with www.google.com can run
the client plugin program like this:
meek-client --url=https://meek-reflect.appspot.com/ --front=www.google.com
The meek-client program by itself has a fingerprintable TLS handshake.
To disguise the TLS part of HTTPS connections, meek-client should be run
with the --helper option pointing at a browser extension that has been
set up separately. How it works is meek-client tells the browser what
URL to request, the browser requests it and returns the payload to
meek-client. The TLS implementation is that of the browser, so it better
blends in with allowed traffic. Browser extensions for Chrome and
Firefox are in the chrome and firefox directories.
Here is a summary of the programs that appear in subdirectories.
meek-client:
The client transport plugin, run by a censored client.
meek-server:
The server transport plugin, run on a Tor relay.
appengine:
Reflector web app that runs on Google App Engine. The reflector simply
copies requests and responses to an instance of meek-server somewhere. A
public instance of the App Engine web app is at
https://meek-reflect.appspot.com/.
chrome, firefox:
Browser extensions for TLS camouflage.
meek-client-torbrowser:
An auxiliary program for the Tor Browser Bundle that runs a copy of
Firefox with the browser extension and then configures meek-client to
use it as a helper.
terminateprocess-buffer:
An auxiliary program used on Windows to assist with cleanup of
subprocesses.
latencytest:
A test program for App Engine that measures roundtrip times to different
destinations.
php:
A php reflector similar to the appengine one above. Ideally, it can be run on
any platform that supports php.

The Meek Protocol has recently been getting a lot of attention since the Tor project made a few blog posts about it. Meek is a censorship evasion protocol that users a tactic called “domain fronting” to evade DPI-based censorship tactics. The idea is that using a CDN such as Google, Akamai, or Cloudflare, you can proxy connections (using the TLS SNI extension) so that if an adversary wanted to block or drop your connection, they would need to block connections to the CDN, like Google; mutually assured destruction. The goal being, a way of connecting to the Tor Network that is unblockable even from nation state adversaries.

SNI and Domain Fronting

SNI is a TLS extension that’s been around for about nine years, and has been implemented in all modern browsers at this point. This is the TLS version of virtual hosting where you send an HTTP request to a server, and inside is a request to another host. Similar to virtual hosting’s host headers, SNI provides a host inside it’s extension during the client hello request:

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Extension:server_name

Type:server_name(0x0000)

Length:21

Server Name Indicator extension

Server Name list length:19

Length:21

Server Name Indication extension

Server Name list length:19

Server Name Type:host_name(0)

Server Name length:16

Server Name:www.antitree.com

This would be a request to https://www.google.com but the server receiving this request would look up the record to www.antitree.com to see if it was fronted, and forward the request to that host.

You should get a response of “I’m just a happy little web server.” which is what the meek-server default response is.

In terms of Internet censorship, the idea of using SNI to proxy a request through a CDN is called Domain Fronting and AFAIK, is currently only implemented by the Meek Protocol. (That being said, the idea can apply to just about any other protocol or tool. I’ve seen other projects use Meek or something like it. ) What Meek provides is a way of using Domain Fronting to create a tunnel for any protocol that needs to be proxied.

Tor and Meek

The Meek Protocol was designed by some of the people involved with the Tor Project as one of the pluggable transports and is currently used to send the entire Tor protocol over a Meek tunnel. It does this using a little bit of infrastructure:

meek-client: This is what a client will use to initiate a tunnel over the Meek protocol

meek-server: corresponding server portion that will funnel requests and responses back over the Meek tunnel

web reflector: In its current form, this takes an SNI request, sees that it is a Meek request, and redirects it to the meek-server. This also makes sure that the tunnel is still running using polling requests.

CDN: the important cloud service that will be fronting the domain. The most common example is Google’s App-Spot.

Meek Browser Plugin: In order to make a meek-client request look like a standard SNI request (same TLS extensions) that your browser would make, a browser plugin is used.

Here’s a diagram of it all wrapped together:

This is how just a request is made to a Tor Bridge Node that’s running the meek-server software. Right now, if you download the latest Alpha release of the Tor Browser Bundle, this is how you could optionally connect using Meek.

Polling

You might notice, that due to the fact HTTP (by design) doesn’t maintain any kind of state to keep a connection open for as long as you would like to tunnel your Tor traffic, the Meek protocol needs to compensate. It does this by implementing a polling method where a POST request is sent from the client to the server at a specified (algorithmic) interval. This is the main way that data is delivered once the connection has been established. If the server has something to send, it’s done in the POST response body, otherwise the message is still sent with a 0 byte body.

Success Rate

You might notice that there are a few extra hops in your circuit and it’s true that there is a decent amount of overhead, but for those in China, Iran, Egypt or the ever-expanding list of other nations implementing DPI based blocking as well as active probing, this is the difference between being able to use Tor, and not. The benefit here is that if you’re watching the connection, you’ll be able to see that a client IP made an HTTPS connection to a server IP owned by Google or Akamai. You cannot see if TLS handshake decide to support the SNI extension, and you cannot see whether or not the client HELLO contained a SNI “server_name” value. Without this, the connection is indistinguishable from a request to say Youtube or Google.

As of now, there does not seem to be a lot (compared to all Tor users) of users connecting over the Meek bridge but it does seem to be increasing in popularity.

Attacks

While no known attacks exist (besides an adversary blocking the entire CDN), there are some potential weaknesses that are being reviewed. One of the interesting ones is if an adversary is able to inject a RST packet into the connection, the tunnel would collapse and not re-establish itself. This is unlike a normal HTTP/S request that would just re-issue the request, and not care. This may be a way of fingerprinting the connections over time but there would be a fairly large cost to other connections in order to perform an attack like this. The other attack of note is traffic correlation based on the polling interval. If the polling interval was static at, for example, 50ms, it would be fairly easy to define a pattern for the meek protocol over time. Of course that’s not the case in the current implementation as the polling interval dynamically changes. The other attacks and mitigations can be found on the Tor wiki page.